The present invention relates to monitoring and ultimately controlling organic additives and particularly the accelerator and suppressor components of the additives in copper plating baths. The copper plating baths can be used to deposit copper interconnections for semiconductor chips.
The present invention is especially advantageous for monitoring copper plating baths that comprise relatively high cupric ion concentrations and/or relatively low sulfuric acid concentration.
Performance of semiconductor circuits can be limited by RC contributions from the on-chip interconnection structure, which may consist of up to 9 or more levels of conductor circuitry built in an insulating material. The R (resistance) factor in RC can be minimized by minimizing the resistivity of the conductive metal (i.e. maximizing its conductivity). To this end, the semiconductor industry is in the process of migrating from the traditional aluminum conductor to the more highly conducting copper in many high-performance products. The method of choice for the deposition of the copper conductor is electroplating, which, when used in combination with chemical-mechanical polishing, has been very successful in producing metal levels with good metal fill of the line and via features of the on-chip interconnection.
With each generation of technology, the feature dimensions become smaller and more challenging to fill even with the versatile electroplating technology. In very small features with high aspect ratios such as greater than 3, the rate of copper plating at the bottom of the feature may be limited by the rate of diffusion of Cu(II) ions in the plating bath. This phenomenon can cause voids in the copper lines; the resulting structures will have poor reliability and low yields in electrical tests. The ability of plating baths to fill very small, high aspect ratio features can be extended by increasing the Cu(II) concentration in the plating bath as disclosed in U.S. patent application Ser. No. 09/684,786 filed Oct. 10, 2000 and entitled xe2x80x9cElectroplated Copper Interconnection Structure, Process for Making an Electroplating Bathxe2x80x9d disclosure of which is incorporated herein by reference.
When the cupric sulfate concentration in a plating bath is increased, the sulfuric acid concentration must be decreased in order to stay below the saturation limit of cupric sulfate. Another motivation for decreasing the sulfuric acid concentration, independent of the cupric sulfate concentration, is to reduce the conductivity of the plating solution. In semiconductor wafer plating, use of low-conductivity plating baths can improve the wafer-scale metal distribution. When 300 mm wafers move into production, it is believed that lower-acid copper plating baths will be introduced into production.
Because of the high value of a very-large-scale integrated circuit chip, the manufacturing processes used in its fabrication must be tightly controlled. For copper plating, one of the most challenging problems is the control of the organic addition agents in the plating bath. The addition agent is a mixture of complicated organic compounds present at low concentrations in the plating bath. There are no truly specific and quantitative methods for separation and analyses of these compounds that can be practically used to control a manufacturing line.
One common monitoring method for organic plating additives is called CVS (for cyclic voltammetry stripping, the electrochemical technique that indirectly determines the solution concentrations of additives through their effects on the kinetics of the electrochemical copper deposition process). The baths for plating of copper wafer circuitry evolved from baths used to plate printed circuit boards. Although plating bath suppliers employ different, proprietary organic additives to control the copper properties, there is not much difference between suppliers in the recommended concentrations of the inorganic components of the plating bath. Typically, the baths have cupric sulfate concentrations on the order of about 0.25M and sulfuric acid compositions on the order of about 2N. They also contain traces of chloride, in the range of 1 to 4 mM.
The high-Cu(II), low-acid plating baths for improved fill as disclosed in U.S. patent application Ser. No. 09/684,786, on the other hand, are very different from these traditional baths in the concentrations of cupric sulfate and sulfuric acid. The cupric sulfate concentration can exceed 1M and the sulfuric acid concentration can be less than 0.25N. Installed CVS methods do not work well in these concentration ranges. Furthermore, it is conceivable that a manufacturing facility would simultaneously operate two plating baths, a traditional copper bath and a high-Cu(II), low-acid bath. It is thus desirable to use similar and compatible analytical methods to control the two kinds of baths.
The present invention is concerned with a method for monitoring the concentrations of the suppressor and accelerator organic addition agents in high-Cu(II) or low-acid copper plating baths. The method of the present invention is compatible both with the available commercial instrumentation and with procedures used for more typical copper plating baths.
Although plating baths may contain 4 or more low-concentration organic components in the additive, a practical approach is to control the additive as if it had only two components. These are usually named according to their affect on the kinetics of the electrochemical copper-deposition reaction. The suppressor is the portion of the additive that slows copper deposition; other common names for the suppressor are polarizer or leveler. The accelerator is the portion of the additive that increases the rate of copper deposition; other common names for the accelerator are depolarizer or brightener.
The present invention is a modification of the CVS analytical procedures for additives in copper plating baths to achieve two purposes: the modified procedures are more accurate for analyses in plating baths with high Cu(II) and/or low acid concentrations, and the operating conditions are compatible with more conventional acid copper plating baths (thus allowing operating of both kinds of baths in one facility with fewer complications in monitoring and control).
The copper plating baths monitored according the present invention comprise organic addition agents, cupric salt, sulfuric acid and hydrochloric acid. The copper plating baths contain a relatively high cupric ion concentration and/or a relatively low sulfuric acid concentration. The organic addition agents include a suppressor component and an accelerator component.
The process of the present invention comprises:
a) obtaining a sample of said bath;
b) diluting said sample with sulfuric acid and hydrochloric acid and optionally with a cupric salt to provide a composition comprising conventional concentrations of cupric ion, sulfuric acid and hydrochloric acid and adjusted concentrations of said organic addition agents of 1/X of their original values in said sample, where X is the dilution factor;
c) obtaining a standard solution of the suppressor component of the organic addition agent having conventional concentrations of cupric ion, sulfuric acid and hydrochloric acid and having a known concentration of the suppressor of 1/X of its target value in the plating bath;
obtaining a first stock solution having the same cupric sulfate, sulfuric acid, and hydrochloric acid concentrations as the diluted sample and optionally also having the accelerator at a concentration of 1/X of its target value in the plating bath;
obtaining a second stock solution having the same cupric sulfate, sulfuric acid, and hydrochloric acid concentrations as the diluted sample and also having a known amount of an electrochemically suppressing chemical;
performing a dilution titration CVS technique for determining the concentration of said suppressor component of the additive in the plating bath using said first stock solution and standardizing the analysis with said standard solution of the suppressor;
performing a standard-addition CVS technique for determining the concentration of said accelerator component of the additive in the plating bath, using said second stock solution and standard additions of the accelerator.